1. Field of the Invention
The present invention relates generally to solder joints. More particularly, the present invention relates to a method and device to elongate a solder joint.
2. Description of the Related Art
A solder joint is formed when a solder deposit between two substrates is subjected to reflow soldering. During reflow soldering, the solder deposit is heated such that it melts and wets a solderable surface on a substrate. The solder deposit solidifies upon subsequent cooling, forming the solder joint.
Solder joints serve two functions in an electronic assembly; they provide mechanical support to hold various components in the electronic assembly together and electrical support to form electrical connections within a circuit. Examples of solder joints include chip-to-interposer connections such as controlled collapse chip connections (C4 joints), package-to-board connections in, for example, Surface Mount Technology (SMT), and chip-to-board connections such as in chip-on-board (COB) technology.
During field application, the electronic assembly undergoes a temperature cycle each time it powers on or off. Due to differences in coefficients of thermal expansion (CTEs), substrates in the electronic assembly expand and contract to different degrees during the temperature cycle. This differential expansion and contraction of the substrates results in a movement of one end of a solder joint relative to another, which puts a strain on the solder joint. Because the electronic assembly undergoes numerous temperature cycles over its service life, the solder joints are subjected to repeated applications of strain, resulting in fatigue failure of the solder joints, which shortens the service life of the electronic assembly.
Studies have shown that the fatigue life of a solder joint, that is, the number of applications of strain that a solder joint can sustain before fatigue failure, can be improved by elongating the solder joint. An elongated solder joint is more compliant and is therefore better able to absorb the strain caused by the temperature cycling. Consequently, various methods to elongate a solder joint have been proposed.
One such method is disclosed in U.S. Pat. No. 4,545,610 issued to Lakritz, et al, wherein a solder extender on a substrate is positioned over a solder mound on a semiconductor chip and reflowed to form an elongated solder joint. A spacer is used to maintain a predetermined spacing between the semiconductor chip and the substrate.
A drawback of this method is that it involves numerous processing steps. Each processing step increases the complexity of the manufacturing process and adds to the cost of production. In particular, the processing steps involving the use of vapour deposition techniques to form the solder extender on the substrate and then to deposit a layer of low melting metal onto a top surface of the solder extender make this method of elongating a solder joint expensive.
In addition, because the probability of defects occurring in the solder joint increases with each processing step, the reliability of an elongated solder joint formed with this method is compromised.
Another method for elongating a solder joint is disclosed in U.S. Pat. No. 5,968,670 issued to Brofman, et al. In this method, the solder joint is elongated when a spring, restrained in a compressed state by solder, is released during solder reflow.
Unfortunately, this method also involves numerous processing steps which, as discussed previously, increase the complexity of the manufacturing process and the cost of production, and reduce the reliability of the solder joint.
Additionally, because some of the components involved in this method are of an infinitesimal dimension, the handling of these components poses a problem. For example, it is difficult to manipulate the tiny spring into a suitable orientation for insertion into a minute cavity in a graphite boat mold to form an expandable solder bump. This difficulty in the handling of the components contributes to the complexity of the process.
In view of the foregoing, it is desirable to have a method for elongating a solder joint that involves a minimal number of processing steps. It is also desirable to have a method that does not involve the handling of minute components.